Field of the Invention
The present embodiments herein relate to the field of mass spectrometry, and more particularly the present embodiments herein relate to the use of a Time of Flight (TOF) instrument in cooperation with a 2D linear multipole configured to receive digitally manipulated waveforms so as to trap, isolate and energize the ions of interest to provide collision induced tandem mass spectrometry MSn and high sensitivity.
Discussion of the Related Art
Mass spectrometry is one of the most common and most important tools in chemical analysis and became a key technique in the discovery of the electron and the isotopes. The analysis of organic compounds is especially challenging as such compounds cover a wide mass range from about 15 amu up to several hundred thousand amu, wherein the compounds themselves are often fragile and non-volatile.
In general, a mass spectrometer includes an ion source, a mass analyzer and some form of one or more detectors. As part of the function of the ion source, sample particles are ionized with techniques that can include chemical reactions, electrostatic forces, laser beams, electron beams, or other particle beams. The resultant ions are subsequently directed to one or more mass analyzers that separate the ions based on their mass-to-charge ratios. The separation can be temporal, e.g., in a time-of-flight analyzer (TOF), spatial e.g., in a magnetic sector analyzer, or in a frequency space, e.g., in ion cyclotron resonance (ICR) cells. The ions can also be separated according to their stability in a multipole (e.g., quadrupole), an ion trap or an ion guide. The separated ions are detected by detectors so as to provide data that enable the reconstruction of a resultant mass spectrum of the sample particles.
As part of the directing of the particles within a mass spectrometer, the ions are guided, trapped or analyzed using magnetic fields or electric potentials, or a combination of magnetic fields and electric potentials. For example, static electric fields are used in time of flight instruments and electrostatic traps, like the ORBITRAP™, static magnetic and static electric fields are used in ICR cells, and static and dynamic multipole electric potentials are used in multipole traps such as, two-dimensional (2D) quadrupole traps or three-dimensional (3D) quadrupole ion traps. However, while a (3D) quadrupole ion trap, e.g., Paul trap, forms a true 3D trapping potential it has only a limited space charge capacity.
With respect to linear 2D multipole traps, such devices, which can be operated as collision cells, often include multipole electrode assemblies, such as quadrupole, hexapole, octapole or greater electrode assemblies that include four, six, eight or more rod electrodes, respectively. The rod electrodes are arranged in the assembly about an axis to define a channel in which the ions are confined in radial directions by a 2D multipole potential that is generated by applying radio frequency (“RF”) voltages to the rod electrodes. The ions are traditionally confined axially, in the direction of the channel's axis, by DC biases applied to the rod electrodes or other electrodes such as plate lens electrodes in the trap. Additional AC voltages can be applied to the rod electrodes to excite, eject, or activate some of the trapped ions.
In MS/MS (e.g., MSn) experiment using desired multipole (e.g., quadrupole) structures, selected precursor ions are often first isolated or selected, and next reacted or activated to induce fragmentation to produce product ions. Mass spectra of the product ions can be measured to determine structural components of the precursor ions. Typically, the precursor ions are fragmented by collision activated dissociation (“CAD”) in which the precursor ions are kinetically excited by electric fields in an ion trap that also includes a low pressure inert gas. The excited precursor ions collide with molecules of the inert gas and may fragment into product ions due to the collisions.
Since becoming commercially available in the mid 1990's, quadrupole time-of-flight mass spectrometers (Q-TOF-MS) have advanced through automation of instrument control and data processing and continued improvements in mass resolution, accuracy and sensitivity. These improvements permit Q-TOF-MS to be applied to biological samples using atmospheric sampling and ionization techniques such as nanospray, microspray and atmospheric pressure chemical ionization (APCI). Their rapid speed of analysis permits them to be used as detectors for liquid chromatography at high flow-rates. Another key feature is their ability to perform such MS/MS experiments with combined high sensitivity and high mass accuracy for both precursor and product ions.
One of the drawbacks of the Q-TOF-MS is the ion sampling process. Ions from a continuous atmospheric pressure source are formed into a continuous beam with only a small portion being sampled into the flight tube for mass analysis during a “scan”. In theory, the percentage of the ion beam sampled into the flight can be maximized by increasing the sampling frequency and optimizing the delay and duration of the pusher pulse. It is claimed that the ion sampling duty cycle can range between 5 and 30% depending on the m/z range of ions and the instrumental parameters. In practice, users do not generally optimize the sampling duty cycle for each new sample and range. As a result, the fraction of analytes detected to the analytes injected into the TOF is usually significantly smaller than projected by the optimized instrument duty cycle. This presents a significant sensitivity loss.
Another consequence of the ion sampling process is the inability to collect and concentrate analyte ions. The solution concentration has to be within a specific range in order to produce an optimal response from the detector. This represents a challenge in protein analysis that stems from sample complexity. For example, protein concentrations in human blood plasma can vary by as much as 10 orders of magnitude. The dynamic range of commercial Q-TOF-MS systems is claimed to be approximately 5 orders of magnitude under the best conditions. Consequently, there is a need to improve the analyzable concentration range.
The ion trap mass spectrometer (ITMS), such as the linear 2D ion trap as discussed briefly above, on the other hand has the ability to collect, isolate and concentrate ions. It is the ability to control the number and range of ions being analyzed and the ability to perform MSn that make ion traps good instruments for quantitative analysis. Automatic gain control makes ion traps useful for quantitation by adjusting the number of ions in the trap to maintain a linear detector response and negate space charge effects. They are also fast and sensitive enough to be used as detectors for chromatography. The resolving power of ion traps depends mostly on scan speed, with higher resolving power achieved at slower scan speeds.
Accordingly, a need exists for improved methods and configurations to capitalize on the traits of the TOF-MS and ITMS to obtain a much more powerful instrument. The embodiments disclosed herein is directed to such a need.